1. Field of the Invention
The invention relates to a process for preparing malonic esters of the general formula 
where R is C1-10-alkyl, C3-10-alkenyl or aryl-C1-4-alkyl.
2. Background Art
The two customary methods of preparing malonic esters start from derivatives of chloroacetic acid. In one method, an ester of chloroacetic acid is reacted with carbon monoxide an alcohol in the presence of a catalyst based on cobalt carbonyl (German Published Patent Application Nos. 2359963 and 2524389) while in the other method a salt of chloroacetic acid is reacted with cyanide to form cyanoacetate in a first step and this intermediate is then reacted with an alcohol in a second step to convert it into the malonic ester. The latter method in particular is associated with safety and ecological problems owing to the toxicity of hydrocyanic acid and cyanides and the large amounts of waste. The obvious method of preparing esters by direct esterification of the acid with the appropriate alcohol plays no role in this case; conversely, malonic acid is prepared by hydrolysis of malonic esters (or cyanoacetic acid).
It is an object of the present invention to provide an alternative route to malonic esters.
This object is achieved according to the invention by the process of the invention.
It has been found that alkali metal salts of malonic acid can be reacted with halides of the general formula Rxe2x80x94X (II), where R is C1-10-alkyl, C3-10-alkenyl or aryl-C1-4-alkyl and X is chlorine, bromine or iodine, in the presence of water to give the corresponding malonic esters of the general formula: 
where R is as defined above, if a phase-transfer catalyst is present.
Here and in the following, C1-10-alkyl can be any linear or branched, primary, secondary or tertiary alkyl group having 1 to 10 carbon atoms, for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isopentyl, neopentyl, hexyl, heptyl, octyl, nonyl, decyl, etc.
C3-10-alkenyl is a linear or branched alkenyl group having 3 to 10 carbon atoms, in particular one whose double bond is separated from the free valence by at least one saturated carbon atom, for example, allyl, methallyl, 2-butenyl (crotyl), 3-butenyl, 2-pentenyl, etc.
Aryl-C1-4-alkyl is, in particular, a phenyl-substituted C1-4alkyl group such as benzyl, phenethyl or 3-phenylpropyl, where the phenyl group may also bear one or more identical or different substituents such as C1-4-alkyl, C1-4-alkoxy or halogen.
Here and in the following, phase-transfer catalysts are the compounds customarily used for this purpose, in particular quaternary ammonium or phosphonium salts.
As alkali metal salt of malonic acid, preference is given to using disodium malonate.
As halide Rxe2x80x94X (II), preference is given to using a chloride or bromide.
The alkali metal salt of malonic acid is preferably used in the form of an aqueous solution. Particular preference is given to the solutions obtained by catalytic oxidation of 1,3-propanediol in the presence of aqueous alkali metal hydroxide. The preparation of such solutions is described, for example, in German Published Patent Application No. 4107986.
As phase-transfer catalyst, preference is given to using a quaternary ammonium salt. Particular preference is given to tetra-n-C4-10-alkylammonium, benzyltri-n-C1-8-alkylammonium and inethyltri-n-C4-10-alkylammonium halides, where halide is preferably chloride or bromide. Examples which may be mentioned here are tetrabutylammonium and tetrahexylammonium bromides and benzyltributylammonium chloride.
The process of the invention is advantageously carried out at temperatures of 80xc2x0 to 150xc2x0 C., when using low-boiling halides (II) conveniently under superatmospheric pressure.
Apart from water, it is advantageous to use an inert solvent which is not miscible with water. Examples of suitable solvents of this type are relatively unreactive aliphatic or aromatic chlorinated hydrocarbons such as chlorobenzene or ethers such as tert-butyl methyl ether.